Semi-conductor devices have been conventionally produced by sealing a semi-conductor element, such as transistors, IC, and LSI, in a ceramic package, etc., but in recent years, the ceramic package is shifted to a resin sealing using a plastic package comprising an epoxy resin as a main component from the standpoint of cost and mass production. In particular, for the purpose of achieving high-density mounting on a hybrid substrate, efforts have been made to reduce the size and thickness of semi-conductor devices. To this effect, sealing by surface-mount type plastic packages such as PLCC (Plastic-Leaded Chip Carrier), SOP (Small Out-line Package), FP (Flat Package), and the like, has lately attracted attention and been widely applied to practical use.
Since the above-described surface mounting, unlike mounting by pin-insertion type DIP (Dual In-line Package), is a technique such that a package is directly soldered on a substrate, the package is heated on mounting to a temperature in the range between 210.degree. C. and 260.degree. C., which is a melting point of the solder, by vapor phase heating or infrared heating. Therefore, the most serious problem encountered with the surface mount type package is that the moisture-resistant reliability of the resulting semi-conductor device after mounting is greatly reduced. That is, a thermal stress arising from thermal shock at the time of mounting and a stress arising from evaporation of moisture present within the package cause separation between a molding resin and a lead frame or between a chip of a semi-conductor element and a molding resin to thereby form a gap therebetween. As a result, outside moisture penetrates into the inside of the semi-conductor device through the gap to seriously reduce the moisture-resistant reliability after mounting.